1. Field of the Invention
This invention relates to a process for producing a thermoplastic resin which is capable of being molded into article having a very good appearance, by adjusting the pH of a coagulated slurry in producing the thermoplastic resin by coagulating a thermoplastic resin latex prepared by an emulsion polymerization method.
Thermoplastic resins having a high added value, such as impact-resistant resins, heat-resistant resins, weather-resistant resins and the like have heretofore been polymers having a special fine structure called graft polymers, tapered polymers or the like and are usually produced by an emulsion polymerization method. Such thermoplastic resins are usually produced as dry powder by a series of production steps comprising polymerization step, a coagulation step, a washing step, a dehydration step, and a drying step, and some of them are used alone and others are used in admixture with other synthetic resins such as AS resins, vinyl chloride resins or the like.
Among these production steps, the coagulation step and the washing step are, in general, continuously carried out. The coagulation step is a step in which first, a polymer latex is brought into contact with a coagulating solution to prepare a coagulated slurry, which is then heated in 1 to 3 steps to solidify the polymer. On the other hand, the washing step is a step in which the slurry of the solidified polymer is washed with water continuously or by a batchwise continuous operation while it is subjected to solid-liquid separation. These coagulation and washing steps are important steps influencing the physical properties of the resulting polymer. In particular, the coagulation step is a step primarily determining the powder characteristics of the resulting resin powder, and research has been made towards improving the powder characteristics such as bulk specific gravity, dehydrability, fluidity parameter and the like. These researches have been conducted in order to reduce formation of fine powder, which is the main cause of troubles in the handling properties, the transportability, storage or drying cost, and is aimed mainly at obtaining powder consisting of particles which are spherical, have a little large diameter, have a narrow diameter distribution and are firm. As a result of these researches, the powder characteristics could be improved, but the powder particles became firm, so that the effect of kneading by means of a screw extruder, a calender apparatus, or the like at the time of melt molding was correspondingly diminished, and therefore stones or fish eyes were apt to be formed in the surface of the resulting molded article, and there were caused phenomena which were not desirable for the surface appearance. Accordingly, now measures are taken to meet the situation in the following manner; the improvement of the powder characteristics is limited so as to be made only to a certain extent in order to prevent the above-mentioned diminution of the kneading effect, or alternatively the kneading effect diminished by the improvement of the powder characteristics is supplemented by other means, for example, the employment of a kneading machine of high performance. In the washing step, it is necessary to remove a residual coagulant, residual polymerization auxiliaries, residual monomers or the like by washing. By this removal, there are solved problems in the physical properties and the operations, such as thermal coloration of the resin, fuming at the time of melting of the resin, or the like. In particular, the removal of the residual coagulant is the most important procedure, and a large amount of washing water as much as 5 to 30 times weight of the polymer to be washed is usually used. However, the employment of such a large amount of water is not desirable from the viewpoint of the conservation of natural resources and the environment.
2. Description of the Prior Art
Under these circumstances, it has been made an ultimate object of any technical improvements that in the coagulation step, there are produced particles containing no fine powder which have a high bulk specific gravity, a good dehydrability and an excellent fluidity, and that in the washing step, such a washing step is made unnecessary. As a part of the technical improvements, there have established, with respect to the improvement of the powder characteristics, techniques such as a spray drying method, a spray coagulation method or "Process for Producing Powder Particles" which the present applicant has previously proposed as Japanese Patent Application No. 73,115/81. Powders obtained by the spray drying method and the spray coagulation method among these techniques are formed of microscopically spherical particles and have their powder characteristics improved in their own way, but are still macroscopically fine powders and thus require, for example, a measure to counter problems caused by the fine powders in handling. Further, such powders have many disadvantages in their production from the viewpoint of energy cost, productivity and the like. On the other hand, powder particles obtained by Japanese Patent Application No. 73,115/81 are very good in powder characteristics such as bulk specific gravity, dehydrability, fluidity and the like and contain almost no fine powder. However, the particles are firm, and therefore when used in admixture with other resins, they are difficult to knead together therewith and sometimes cause a problem in the surface appearance of the resulting resin in parts. Therefore, development has heretofore been desired of a technique which meets the opposing requirements of the improvement of the powder characteristics and the improvement of the kneadability.